Magnetic resonance data acquisition systems comprise generally a superconducting magnet for providing a static magnetic field. Gradient coils are provided to focus the magnetic field. The gradient coils and the static magnetic field are used to align nuclei in a desired plane of the sample being imaged or spectrographically studied under MR conditions. A pulsed radio frequency (Rf) MR signal is used to nutate the nuclei. When the Rf pulse ends the nutated nuclei tend to return to the aligned condition. As they are returning they generate free induction decay (FID) signals. It is the FID signals that are most popularly used for imaging purposes.
The coils used for transmitting the Rf signals are also generally used for receiving the FID signals. The Rf frequency used is the LARMOR frequency. The LARMOR frequency as is well known is a function of the particular element under study and the magnetic field strength.
The static magnetic field is generated by the superconducting magnet and the specimen or patient is placed within the bore of the superconducting magnet. The Rf coils or probes, are generally built around the bore, however, it has been found, as can be expected, that better nutation and more efficient reception is obtained when probes, are used for imaging particular sections of the body rather than the main probe. The Rf probes such as a head probe, leg probe, and body probe are Rf coils arranged to be juxtaposed to the patient's body close to the plane or volume being imaged.
Improved images are provided by the probes that are used proximate to the portions of the body being imaged because among other reasons, of a filling factor.
Notwithstanding the more efficient action of the proximate probes, signal to noise ratio of the required data remains critical because of the very small amplitudes of the FID signals. The probes cause an increase in noise, among other things, because of imbalances due to stray capacitance in the proximate probes themselves and because of the variations in the impedance of the coils of the probes introduced by the patient or sample. Thus, different patients have different body impedances and therefore effect the proximate probes differently. In addition to the effect of the individual samples, the distributed capacitance of the probes is variable in that it changes with temperature and relative humidity, among other things. The rooms wherein magnetic resonance data acquisition occur are carefully controlled as to temperature and humidity. Nonetheless there are day to day variations in the distributed capacitance of the proximate probes.
In addition, a basic problem with all large coils is that they normally have low self resonance frequencies. Another problem is the heating or conductive samples in superconducting spectrometers. These problems are treated in the following two articles respectively:
(1) A Large Inductance High Frequency High Q, Series Fused Coil for NMR" by B. Cook and I. Lowe, Journal of Magnetic Resonance 49, pp 346-349, 1982; and
(2) "An Efficient Decoupler Coil Design which Reduces Heating in Conductive samples in Superconducting Spectrometers" by D. W. Alderman and D. M. Grant, Journal of Magnetic Resonance, 36, pp 447-451, 1979.